Delete comment from: Ken Shirriff's blog
The junction isolated bipolar process used for TTL was inherently more radiation resistant than the junction isolated CMOS processes (1) which replaced it so it would make a good existing low cost choice for space electronics. Eventually dielectrically isolated processes replaced junction isolated processes where this was important however I do not know that they were ever used at lower levels of integration. I am amused that they upgraded to FAST (Fast Advanced Schottky TTL); it is a good choice and I wish the whole 74xx series had been implemented in it.
Power supply isolation makes perfect sense between chassis and sometimes even between boards. It allows the power and signal grounds to be separated improving noise immunity and preventing some catastrophic failure modes. An isolated switching power supply also means that a pass element short will not apply high voltage to the circuits.
(1) For instance the photocurrent generated by single radiation event can trigger the parasitic SCR in a common junction isolated CMOS process causing catastrophic destruction. I have seen ESD do this to powered HCMOS logic. Junction isolated bipolar logic does not suffer from this particular problem.
Jan 20, 2020, 10:47:12 PM